The present invention relates to a polyester film for metal sheet laminating, a metal sheet laminated with the film, and a metal container formed from the metal sheet. More particularly, the invention relates to a polyester film for metal sheet laminating and a metal sheet laminated with the film, which are suitable for production of a container by deep drawing and ironing of the metal sheet, and to a metal container formed from the metal sheet.
It is a common practice to apply a solvent-type paint essentially containing a thermosetting resin on interior and exterior surfaces of a metal can for prevention of corrosion. However, formation of a coating film of the solvent-type paint requires heating at a high temperature, and entails evaporation of a large amount of a solvent, thereby presenting problems associated with operation safety and environment. Recently, coating the surfaces of the can with a thermoplastic resin has been proposed as a solvent-free corrosion preventing method. Among thermoplastic resins, polyesters are particularly excellent in formability and heat resistance, so that polyester-based films for metal sheet laminating particularly suitable for applications to cans are now under development.
For coating a metal sheet with a film, a thermoplastic resin is melt-extruded directly on the metal sheet, or a thermoplastic resin film is heat-pressed on the metal sheet directly or with the intervention of an adhesive. Particularly, the use of the thermoplastic resin film is advantageous, because the resin can be easily handled with an excellent operability and the resin film has a highly uniform thickness. Since the use of the intervening adhesive is disadvantageous in terms of environmental consideration and costs, the direct heat-press of the film on the metal sheet is more advantageous and attractive.
A metal can coated with the thermoplastic resin film is produced by laminating a metal sheet such as a steel sheet or an aluminum sheet (including a metal sheet subjected to a surface treatment such as plating) with the thermoplastic resin film, and working the laminated metal sheet into the can.
The thermoplastic resin film for use in such an application should simultaneously satisfy various property requirements. That is, the film is required: (a) to have an excellent heat laminatability with respect to the metal sheet; (b) to ensure an excellent can workability without film separation and occurrence of cracks and pinholes in a can working process; (c) to be free from embrittlement when being subjected to a printing process and a retort sterilizing process after the can working process, and when being stored for a long period of time; and (d) to be excellent in content taste and flavor preserving property.
Several types of polyester films including films imparted with heat laminatability and films formed of polyesters blended or copolymerized with other components for improvement of the can workability have been proposed as the thermoplastic resin film for metal sheet laminating.
(A) Films each formed of a copolymer of polyethylene terephthalate (PET) and a second component, for example, are stated in JP-B-8-19245, JP-B-8-19246 and Japanese Patent Publication No. 2528204.
(B) Films each formed of a blend comprising 99 to 60 wt % of a polyester copolymer containing ethylene terephthalate as a main recurring unit and having a melting point of 210 to 245xc2x0 C. and 1 to 40 wt % of polybutylene terephthalate (PBT) or a copolymer thereof are disclosed in Japanese Patent Publication No. 2851468, JP-A-5-186612 and JP-A-5-186613.
(C) A film comprising 99 to 60 wt % of a polyester copolymer and 1 to 40 wt % of a polyester mainly containing butylene terephthalate and having a melting point of 180 to 223xc2x0 C., and having an intrinsic viscosity of not lower than 0.68 and lower than 0.75 is disclosed in JP-A-10-316775.
(D) A polyester film having an intrinsic viscosity of not lower than 0.75 is disclosed in JP-B-7-84532.
(E) A film of a polyester containing an ethylene terephthalate unit in a proportion of 90 mol % and having a reduction viscosity of 0.8 to 1.1 dl/g is disclosed in JP-A-11-279294.
In the case (A), the films formed of the PET copolymer each have a lower melting point and a lower crystallinity, whereby the heat laminatability and the formability are improved. However, the films are liable to be embrittled when being subjected to a heat treatment and the retort sterilizing process after the can working. Therefore, the shock resistance is disadvantageously reduced.
In the case (B), the films each comprise a PBT-based resin, whereby the embrittlement resistance and the shock resistance after the can working and the heat laminatability are improved. However, the heat laminatability and the adhesion with respect to a metal are insufficient. Therefore, the formability in a high-level working process, particularly in a deep drawing and ironing process, is unsatisfactory.
In the case (C), the film has a low intrinsic viscosity (not lower than 0.68 and lower than 0.75), so that a deformation follow-up property for a severer deep-drawing and ironing process is insufficient. In addition, the crystallization characteristic of the film is unsatisfactory because the resin virtually contains the PBT component in a proportion of not greater than 40 wt %. Therefore, the resistance to the retort process, the long-term storage stability after the process and the shock absorbing property are not always sufficient.
In the case (D), the use of the polyester film having an intrinsic viscosity of not lower than 0.75 is proposed. It is stated that the film is effective for the retort resistance and the shock resistance and for prevention of the deterioration in the taste of the content.
More specifically, a homogeneous copolymer essentially containing PET, polyethylene naphthalate or polycyclohexanedimethylene terephthalate is proposed as the polymer. The film for the can is required to have properties contradictory to each other, e.g., a heat-laminating property, a deformation follow-up property for a severer high-level working process, resistance to adhesion to a working jig and long-term storage stability after the retort process, to satisfy recent high performance requirements. However, it is difficult for the aforesaid homogeneous polymer to satisfy all the property requirements.
In the case (E), the polyester film is proposed, which contains the ethylene terephthalate unit in a proportion of at least 90 mol %, has a reduction viscosity of 0.8 to 1.1 dl/g, and is durable in a deep-drawing and ironing process and less susceptible to whitening when being immersed in boiling water. The polyester film is imparted with a boiling water resistance by employing a polymer containing the ethylene terephthalate unit having a higher resistance to heat and boiling water in a proportion of not smaller than 90 mol %. The polyester film is imparted with the adhesion to a metal by introducing a copolymerizable component in a proportion of not smaller than 3 moles and smaller than 10 moles into the polymer.
However, this polymer also falls within the category of the homogeneous polymer, suffering from the same limitations as in the case (D). Therefore, the polymer cannot sufficiently satisfy the requirement for the deformation follow-up property for a recent high-speed and high-level working process. More specifically, the concentration of the copolymerizable component in the polyester should be increased if greater importance is placed on the deformation follow-up property. As a result, the adhesion of the film to a jig is increased in the can working process, thereby deteriorating the productivity, and the retort resistance and long-term storage stability of the can.
On the contrary, the inventors of the present invention previously proposed biaxially stretched films each comprising 90 to 45 wt % of a PBT or PBT-based polyester (I) and 10 to 55 wt % of a PET or PET-based polyester (II) (JP-A-9-194604, JP-A-10-110046). The films thus proposed each have a high crystallinity, and allow for heat-press at a relatively low temperature. In addition, the resulting laminated metal sheets are excellent in workability. Further, the films are not embrittled even during a retort sterilizing process and after long-term storage, and are excellent in shock resistance.
Further, WO95-15993 proposes a polyester film composed of a polyester composition obtained by homogeneously mixing a polyethylene terephthalate containing an ethylene terephthalate unit as a main recurring unit with a polybutylene terephthalate containing a butylene terephthalate unit as a main recurring unit. The polyester film is designed so that the crystallization temperature, secondary transition temperature and plane orientation factor thereof respectively fall within predetermined ranges. Therefore, the polyester film is free from white spots even if being subjected to the retort process after being melt-laminated on a metal sheet.
With recent increasing demands for a higher can producing rate, a greater can volume and a smaller can thickness, there is a tendency toward an increase in the working deformation ratio of the metal in the deep-drawing and ironing process, and toward an increase in friction occurring with respect to the working jig. Even if the film proposed by the inventors is applied on a can body to be subjected to severe deformation, the film suffers from whitening or micro-cracks occurring due to slight fluctuations in conditions for the production of the laminated metal sheet or in conditions for the production of a final can product.
In addition, a residual strain occurring in the film due to an increased working ratio may cause separation of the film due to partially insufficient adhesion to the metal, thereby arousing concern about protection of the content of the can. Further, the film may adhere to a deep-drawing/ironing jig in the can production process, thereby causing rupture of the can body. Hence, it is demanded to improve the film so that the film is able to maintain its properties even under severer working conditions. Since cans are extensively utilized for cold soft drink, there is a great demand for improvement of the shock resistance of the laminate film to withstand an external impact force exerted on the cans when the cans are dropped or during the processing or distribution of the cans. Since the cans are stored for a long period of time, long-term stability is also an important property of the film for long-term cold storage of the cans and hot storage of the cans in winter. As film-laminated cans are more extensively utilized, the film and the film-laminated cans are now required to provide more advanced performance. Hence, there is a demand for immediate development and improvement of a film suitable for the film-laminated cans.
It is an object of the present invention to provide a polyester film for metal sheet laminating, which is excellent in heat laminatability with respect to a metal sheet, in can workability, particularly high-level workability for deep drawing and ironing, in shock resistance and in content taste and flavor preserving property, and is suitable for a film-laminated metal can, and to provide a laminated metal sheet and a metal container formed from the laminated metal sheet.
To achieve the aforesaid object, there is provided a polyester film for metal sheet laminating, the polyester film comprising a blend of a polyester (I) consisting of polybutylene terephthalate or consisting essentially of polybutylene terephthalate and a polyester (II) consisting of polyethylene terephthalate or consisting essentially of polyethylene terephthalate, the polyester (I) being present in the film in a proportion of 80 to 40 wt %, the polyester (II) being present in the film in a proportion of 20 to 60 wt %, the polyester (I) having a melting point of 200 to 223xc2x0 C., the polyester (II) having a melting point of 230 to 256xc2x0 C., the film, as a whole, having an intrinsic viscosity of not lower than 0.75.
With this arrangement, the polyester film for metal sheet laminating can be provided which is excellent in heat laminatability, in formability, particularly high-level workability for deep drawing and ironing and in shock resistance and retort resistance after the working, and suitable for coating a metal can.